In the case of a new aircraft in which CFRP (carbon fiber reinforced plastic) is used for the main wings and/or airframe structure, if a main wing is struck by lightning, discharge (=voltage spark) or spark (=thermal spark or arc) may be generated at a fastener (a fastening bolt made of metal) attached to the main wing of CFRP. In such a case, there arises a possibility that fuel stored in the wing catches fire and explodes. Therefore, restraint of generation of such discharge has become important. It is said that if discharge energy exceeds 200 μJ (threshold), catching fire occurs (there is a standard on this).
Such a new material requires a test for evaluating the effectiveness and soundness of developed techniques. A lightning resistance test is one such test. In a lightning resistance test for aircraft, evaluation is made as to whether or not discharge or spark is generated upon application of lightning impulse current. In this test, a large current which has a waveform imitating that of a lightning stroke and whose peak is several 10s of kA to 200 kA is applied to a sample.
Conventionally (according to the standard), an image of emitted light is captured by a film camera, and evaluation is made as to whether or not light emission is recorded on a film when the input energy is 200 μJ. Therefore, it has been necessary to install a camera at a location where the entirety of an object to be observed can be seen and to install a plurality of cameras so as to perform evaluation at a plurality of locations. Accordingly, it has been difficult to secure a location (s) where a camera or a plurality of cameras are installed. Although the ISO sensitivity of film and the F value of lens are designated, the conventional method has problems; for example, quantitative evaluation of the energy of emitted light is theoretically difficult. In the case where a camera cannot be installed, a gas mixture containing hydrogen gas is charged into a measurement object and an explosion test is carried out. However, this method has a problem of unable to identify the location of any explosion.
Therefore, there has been demanded development of a light, emission detection technique which determines, in the lightning resistance test for aircraft, whether or not discharge or spark is present and determines the energy of the discharge or spark if any. Patent Document 1 discloses a technique of detecting light emitted as a result of partial discharge within gas insulation equipment through use of a light emission detector (a light-receiving element) and evaluating the detected light although the technique is not for the lightning resistance test of aircrafts. For the light emission detector, there is used a light-receiving element whose detection sensitivity is high for the light, emitted as a result of partial discharge (in particular, a wavelength range of 300 nm to 600 nm) for example, a photomultiplier tube (PMT) or a photodiode of high sensitivity. The output of the light-receiving element is observed through use of a measuring device for example, a waveform observing device such as an oscilloscope).
In such a lightning resistance test, discharge or spark from a sample (in the case of a sample with a fastener F, discharge or spark from the vicinity of the fastener) is measured. For the measurement, unlike the case where light emitted as a result of partial discharge within gas insulation equipment is detected, a large current which has a waveform imitating that of a lightning stroke and whose peak is several 10s of kA to 200 kA is applied to a sample. Therefore, the lightning resistance test must be carried out in a poor electromagnetic noise environment.